Project Summary Young adults have the highest prevalence of waterpipe tobacco smoking (WTS) of any age group in the U.S. Extensive evidence shows that they consistently misperceive WTS as less harmful and less addictive than cigarette smoking, and these misperceptions are positively associated with WTS. Despite these widely held misperceptions, WTS is associated with serious acute and chronic harms, even among infrequent users. In addition, WTS is prospectively associated with cigarette smoking initiation putting young adults at greater risk for cancer. Considerable evidence shows that cigarette warnings effectively convey health information to consumers, resulting in increased motivation to quit and quit attempts. Waterpipe warnings have the potential to convey accurate information about health harms to consumers, but the currently-mandated text warning, focused solely on nicotine addiction, is likely to have limited impact. The goal of this project is to develop warnings to promote greater understanding of harms and to reduce WTS behavior among young adults. We will use a systematic, evidence-driven, rigorous approach to develop waterpipe warnings and test these newly- developed warnings in two contexts: (1) on waterpipe tobacco packaging; and (2) in an ecologically-valid waterpipe caf setting. In Aim 1, we will develop text and pictorial waterpipe tobacco warnings to effectively communicate a broad range of health harms. We will first develop 30 text warnings based on the most robust science; then vet warnings with experts in WTS epidemiology, toxicity, and regulation. We will next conduct a national survey to determine the five most effective text warnings, which will then each be paired with three different images creating 15 pictorial warnings. We will conduct a second national survey to determine the most promising image for each of the five text warning statements, resulting in five text and five corresponding pictorial warnings to be carried forward to Aim 2. In Aim 2, we will evaluate the effect of text or pictorial warnings on waterpipe tobacco packaging on WTS intentions using a nationally representative sample of 1,500 young adults and will determine the two text and two corresponding pictorial warnings with the highest perceived effectiveness for use in Aim 3. In Aim 3, we will determine the impact of text and pictorial warnings displayed at waterpipe cafs on WTS behavior. We will conduct a three-arm (text, pictorial, no warning control) cluster randomized crossover trial in 18 waterpipe cafs to determine the effect of warning type on young adults' WTS behavior using an objective, biological marker of waterpipe tobacco consumption (expired carbon monoxide). This study will contribute to a growing body of research on how best to design health warnings to discourage WTS, and will identify important behavioral antecedents, which may include negative affect, cognitive elaboration, perceived risks, and knowledge. The proposed research has the potential to make a significant contribution towards cancer prevention by reducing WTS through the development and testing of warnings that could influence local, state, and federal policies.